(1) Field of the Invention
This invention relates to an image sharpness processing method and apparatus, and a storage medium storing a program, used, for example, in a plate-making process for enhancing or suppressing tone variations in outline portions of an original image.
(2) Description of the Related Art
The principle of sharpness processing will be described with reference to FIG. 1.
Assume that a particular area of an original image has an image signal S (i.e. an original signal S) as shown in FIG. 1(a). Key pixels in the particular area are successively selected, and an average signal is obtained from color component signals, for example, of 5.times.5 pixels surrounding each key pixel. This corresponds to an unsharp signal U as shown in FIG. 1(b). Next, the unsharp signal U is subtracted from the original signal S to obtain an unsharp mask signal (S-U) as shown in FIG. 1(c). The unsharp mask signal (S-U) is added to the original signal S to obtain a signal enhancing outline portions as shown in FIG. 1(d). At this time, the unsharp mask signal (S-U) is multiplied by a parameter k to adjust a strength level of processing. An image with enhanced sharpness is obtained if parameter k is larger than zero. However, if -1&lt;k&lt;0, the image will be something between the original signal S in FIG. 1(b) and unsharp signal U in FIG. 1(b), i.e. a blurred image.
It has been conventional practice to effect such sharpness processing with uniform strength for an entire original image. The following inconvenience results from sharpness processing effected with uniform strength for an entire original image.
When an object with depths, such as a landscape, is photographed with a camera, near areas in focus appear with clear outlines while distant areas are blurred. Images in the proper perspective are often obtained with a varying degree of blur according to distances of objects.
However, when sharpness is enhanced uniformly for an original image obtained by photographing an object with depths, not only near areas but distant areas appear with clear outlines. This ruins the perspective effect of the original image.